Influenza remains an important preventable cause of morbidity and mortality worldwide. The primary strategy to attenuate the impact of influenza epidemics is vaccination, but vaccine effectiveness is suboptimal. Additionally, in the event of a pandemic, vaccines may not be available. Alternative methods to prevent the spread of influenza are needed but little is known about the spread of infection in households and communities in our modern lifestyle. Current surveillance activities do not allow for the characterization of the transmission dynamics of influenza in the community. Understanding the contemporary patterns and timing of influenza infection within households and communities is the foundation for preventative interventions such as home- quarantine, prophylactic use of antivirals, social distancing, school-closures, and vaccination. It is believed that up to 30% of influenza transmission occurs in households. Our understanding of influenza transmission in households and communities has been largely based on studies that were done several decades ago. Diagnostic tests have changed and current molecular and sequencing techniques provide new opportunities to study influenza transmission. Similarly, social interaction patterns within households and communities have likely changed during the last decades, and the widespread availability and use of personal electronic devices may have further modified the dynamic of human contacts within households. Thus, transmission patterns may have changed. Prospective efficient household transmission studies are needed to understand the current dynamics of influenza transmission within households and to identify complementary strategies for disease prevention and control. During three consecutive years, we will conduct a case-ascertained study to determine the transmission of influenza in households, with the following specific aims: 1. To determine the risk of secondary influenza transmission in households over three consecutive influenza seasons; 2. To determine the effectiveness of influenza vaccination (VE) against secondary influenza transmission in households over three consecutive influenza seasons.